[PATCH v3 23/23] Makefile: move ".SUFFIXES" rule to shared.mak

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

This was added in 30248886ce8 (Makefile: disable default implicit
rules, 2010-01-26), let's move it to the top of "shared.mak" so it'll
apply to all our Makefiles.

This doesn't benefit the main Makefile at all, since it already had
the rule, but since we're including shared.mak in other Makefiles
starts to benefit them. E.g. running the 'man" target is now ~1.3x
faster:

    $ git -c hyperfine.hook.setup= hyperfine -L rev HEAD~1,HEAD~0 -s 'make -j8 -C Documentation man' 'make -C Documentation man' --warmup 10 -M 10
    Benchmark 1: make -C Documentation man' in 'HEAD~1
      Time (mean ± σ):      87.2 ms ±   1.0 ms    [User: 79.3 ms, System: 10.8 ms]
      Range (min … max):    86.3 ms …  89.7 ms    10 runs

    Benchmark 2: make -C Documentation man' in 'HEAD~0
      Time (mean ± σ):      64.5 ms ±   0.6 ms    [User: 54.5 ms, System: 13.0 ms]
      Range (min … max):    63.8 ms …  65.7 ms    10 runs

    Summary
      'make -C Documentation man' in 'HEAD~0' ran
        1.35 ± 0.02 times faster than 'make -C Documentation man' in 'HEAD~1'

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 Makefile   | 2 --
 shared.mak | 5 +++++
 2 files changed, 5 insertions(+), 2 deletions(-)

diff --git a/Makefile b/Makefile
index 1041efb6e5e..d2cc6865a4e 100644
--- a/Makefile
+++ b/Makefile
@@ -2460,8 +2460,6 @@ ASM_SRC := $(wildcard $(OBJECTS:o=S))
 ASM_OBJ := $(ASM_SRC:S=o)
 C_OBJ := $(filter-out $(ASM_OBJ),$(OBJECTS))
 
-.SUFFIXES:
-
 $(C_OBJ): %.o: %.c GIT-CFLAGS $(missing_dep_dirs) $(missing_compdb_dir)
 	$(QUIET_CC)$(CC) -o $*.o -c $(dep_args) $(compdb_args) $(ALL_CFLAGS) $(EXTRA_CPPFLAGS) $<
 $(ASM_OBJ): %.o: %.S GIT-CFLAGS $(missing_dep_dirs) $(missing_compdb_dir)
diff --git a/shared.mak b/shared.mak
index 4862df1607a..a20193b0c8f 100644
--- a/shared.mak
+++ b/shared.mak
@@ -9,6 +9,11 @@
 %:: s.%
 %:: SCCS/s.%
 
+## Likewise delete default $(SUFFIXES). See:
+##
+##     info make --index-search=.DELETE_ON_ERROR
+.SUFFIXES:
+
 ### Flags affecting all rules
 
 # A GNU make extension since gmake 3.72 (released in late 1994) to
-- 
2.34.0.795.g1e9501ab396